Step 1: Easy Making of the Power Bank

Once you have all the materials you should be sure that all of them are good. With help of your DMM check voltage across battery and the solar panel. Check the diode and the switch as well. Boost converter circuit is the most delicate part here, be careful with it. The battery had a built-in circuitry to tackle over charging, low voltage cutoff, reverse and short circuit protection. The battery used here is a tablet Phone battery since it is 2 to 3 mm thick you can put it in a small sleeve in your bag or even cut out of a portion of an old diary or you have a more creative idea! try it. You may even take the 3.7v battery of any cellphone.

The DPDT Slide switch has battery connected to the center terminals. The other terminals are connected with solar panel on one side with a diode and the other side with the boost converter PCB. Be sure that the polarities are in the same side of the switch i.e, the top row has only +ve and the bottom rows are -ve connected. Strip the ends of all the wires and should be dipped in flux and well soldered for easy soldering to the switch. If you are not clear with the wiring then start from the switch. The switch has 6 terminals and to any one end, solder the diodes cathode (end with the silver band), Now to the open end of diode, solder the Solar panels +ve (red wire), once done you will find remaining 5 terminals to be soldered. With respect to the diode we see that there are 2 free terminals to one side and 1 free terminal other side. Now solder the solar panels -ve (black wire) to the 1 free terminal. With respect to the connection made you have to connect the battery to the next terminals(center terminals) and then the Boost converter with all the +ve and -ve aligned to the respective rows. If not done properly there are chance of fire and damages, please be careful!

Step 2: Pros and Cons

I am an known DIYer among my huge community and my many brought this for their smartphones and various other USB powered device . I just put some of my friends reviews below here for your reference.

Advantages of DIY powerbank :- This works WELL!! most of the Chinese Solar power banks don't work well. Some have poor power back up and some takes a weeks time to charge the inbuilt battery, all of them come with poor quality Chinese battery and bad solar panel. DIY project costs only 1/3rd of the cheap Chinese bad stuff. A friend with an android phone got it charged up four times when his phone went below 50%. Another friend powered a USB Fan for few hours and then stopped it fearing of damages. One friend sewed it to a travel bag and all you need to do that, get some piece of plywood and bag material.

Disadvantages of DIY powerbank :- Looks very odd, you need a good enclosure. Cannot charge and discharge at the same time. If left without proper packing, wires come apart. Becomes hot when driving large loads(usual with most of the power banks). Every time you don't get the same parts to make one.

Hi Guptaji, pls read the instructable. Its is a very simple circuit, I had worked on this few months ago and I don't have anything else other than what is posted here. In simple words i will explain - you need a battery connected to the solar panel in parallel, a diode will keep the current flow in the direction of the battery. The boost circuit pcb needs to be connected to the battery. The Switch doesn't have any importance and you can avoid it.